1. Field of the Invention
This invention relates to electro-optic sampling probers that are used to measure waveforms of measured signals by using electro-optical crystals.
This application is based on Patent Application No. Hei 10-340823 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
The electro-optic sampling probers utilize electro-optic probes, which operate as follows:
Electric fields caused by measured signals are applied to electro-optical crystals, on which laser beams being produced based on timing signals are incident. Waveforms of the measured signals are measured for observation in response to polarization states of the laser beams, which are changed in response to the electric fields. Herein, by making the laser beams in a pulse-like form for sampling of the measured signals, it is possible to measure the waveforms with a very high resolution with respect to time.
As compared with the conventional probers using electric probes, the electro-optic sampling probers (abbreviated by xe2x80x9cEOS probersxe2x80x9d) draw considerable attention of engineers and scientists because of some advantages as follows:
(1) At measurement of signals, the EOS probers do not require ground lines. So, it is possible to perform measurement with ease.
(2) Tip ends of the EOS probers are insulated from the circuitry, so it is possible to realize high input impedance. Therefore, the EOS probers do not substantially disturb states of measuring points.
(3) Because the EOS probers utilize optical pulses for measurement, it is possible to perform measurement of a broad band, a frequency range of which is in Giga-Hertz (GHz) order.
(4) The EOS probers are designed such that electro-optical crystals are brought in contact with wafers of integrated circuits having wires, on which laser beams are converged. So, the EOS probers are capable of performing measurement with respect to fine wires, which cannot be brought in xe2x80x9cphysicalxe2x80x9d contact with metal pins.
For convenience sake, the following description uses a specific unit of nano-meter (nm) for dimension of the wavelength of light.
Now, an example of a construction of an EOS prober will be described with reference to FIG. 6. In FIG. 6, an IC wafer 1 is connected to the external (i.e., device or system, not shown) by way of power lines and signal lines. A electro-optical element 2 is configured using an electro-optical crystal. An objective lens 3 converges beams to be incident on the electro-optical element 2. A prober unit 4 is equipped with a dichroic mirror 4a, a half mirror 4b and a reflecting mirror 4c. An EOS optical module (or EOS optical system) is constructed using photodiodes, polarization beam splitters and wavelength plates, all of which are not shown.
An optical fiber 7 is equipped with a fiber collimator 7a at a terminal end thereof. A light source (i.e., laser) 8 supplies the EOS optical system 6 with laser beams. Herein, outgoing beams of the laser 8 have a wavelength of 1550 nano-meter (nm) at maximum intensity. A halogen lamp 9 illuminates the IC wafer 1 being subjected to measurement. Incidentally, the EOS prober of FIG. 6 does not necessarily use the halogen lamp 9 for illumination of the IC wafer 1. That is, it is possible to use other lamps such as the xenon lamp and tungsten lamp.
An infrared camera (or IR camera) 10 makes confirmation with respect to the positioning for convergence of beams on the IC wafer 1. Images created by the IR camera 10 are displayed on a screen of a monitor 10a. The IR camera 10 has light-receiving sensitivity in a range of wavelengths between 500 nm and 1800 nm. An absorption stage 11 absorbs the IC wafer 1 to be fixed in position. The absorption stage 11 is capable of making small movements in x-axis, y-axis and z-axis directions, which are perpendicular to each other.
Next, optical paths of laser beams radiated from the laser 8 will be described with reference to FIG. 6. In FIG. 6, optical paths of laser beams in the prober unit 4 are designated by reference symbols A, B and C respectively.
First, laser beams output from the laser 8 pass through the optical fiber 7, in which they are converted to parallel beams by the fiber collimator 7a. The parallel beams pass through the EOS optical system 6 and are then introduced into the prober unit 4 as its incoming beams. In the prober unit 4, the incoming beams propagate along the optical path A. The incoming beams are reflected by the reflecting mirror 4c, by which they are changed in propagation direction by an angle of 90 degrees. So, reflected beams propagate along the optical path B. Herein, the reflecting mirror 4c is a surface mirror of a full reflection type, which is made by depositing aluminum material on a glass surface.
The reflected beams, corresponding to the laser beams reflected by the reflecting mirror 4c, are further reflected by the dichroic mirror 4a, wherein they are changed in propagation degree by an angle of 90 degrees. So, further reflected beams being reflected by the dichroic mirror 4a propagate along the optical path C. The objective lens 3 converges such further reflected beams onto an opposite surface of the electro-optical element 2 that is placed being opposite to face with a surface of the IC wafer 1, wherein the electro-optical element 2 is arranged on wiring (or wires) of the IC wafer 1. FIG. 7 shows an example of an optical characteristic in transmission of the dichroic mirror 4a with respect to wavelength, wherein a horizontal axis represents xe2x80x9cwavelengthxe2x80x9d while a vertical axis represents xe2x80x9ctransmission factorxe2x80x9d. According to the optical characteristic shown in FIG. 7, the dichroic mirror 4a allows only 5% of light to transmit therethrough while reflecting 95% of light with respect to wavelength of 1550 nm. For this reason, 95% of the laser beams radiated from the laser 8 are reflected by the dichroic mirror 4a, wherein they are changed in optical path by an angle of 90 degrees.
A dielectric mirror is deposited on the opposite surface of the electro-optical element 2, which faces with the IC wafer 1. Laser beams being reflected by the dielectric mirror 4a are converted to parallel beams by the objective lens 3. Then, the parallel beams propagate back along the optical paths C, B, A in turn and are returned to the EOS optical system 6. Those beams are incident on photodiodes (not shown) within the EOS optical system 6.
Next, a description will be given with respect to operations in positioning of the IC wafer 1 in connection with the halogen lamp 9 and the IR camera 10. Specifically, the following description is given with respect to optical paths for light radiated from the halogen lamp 9 and positioning operations of the IC wafer 1. In FIG. 6, the optical paths along which the light of the halogen lamp 9 propagate are designated by reference symbols D, E and C respectively.
The light radiated from the halogen lamp 9 propagate along the optical path D and is incident on the half mirror 4b. The half mirror 4b reflects the light by an angle of 90 degrees, so that reflected light propagate along the optical path E. The reflected light propagate straight through the dichroic mirror 4a along the optical path C so as to illuminate the IC wafer 1. Herein, the half mirror 4b is designed such that reflected light and transmitted light coincide with each other in intensity.
FIG. 8 shows an optical characteristic in radiation of the halogen lamp 9 with respect to wavelength, wherein a horizontal axis represents xe2x80x9cwavelengthxe2x80x9d while a vertical axis represents xe2x80x9clight intensityxe2x80x9d. FIG. 8 shows that the halogen lamp 9 radiates light in a range of wavelengths between 400 nm and 1650 nm.
The IR camera 10 picks up an infrared image with respect to a part of the IC wafer 1 within a visual field of the objective lens 3 being subjected to illumination by the halogen lamp 9. Such an infrared image is displayed on the screen of the monitor 10a. A human operator slightly moves the absorption stage 11 or the prober unit 4 while looking at the image displayed on the screen of the monitor 10a. Thus, the human operator is capable of adjusting a positional relationship between the absorption stage 11 and the prober unit 4 such that the wiring of the IC wafer 1, which is a measured subject, comes into the visual field.
The laser beams, which are radiated from the laser 8 and are introduced into the prober unit 4, are reflected by the surface of the electro-optical element 2 above the wiring of the IC wafer 1. Then, reflected beams are (partially) transmitted through the dichroic mirror 4a and are input to the IR camera 10. So, the IR camera 10 produces an image corresponding to the transmitted beams of the dichroic mirror 4a. By watching such an image, the human operator adjusts the absorption stage 11 or the prober unit 4 in position such that the laser beams are converged at a point on the surface of the electro-optical element 2 above the wiring, which is subjected to measurement. As described before, the dichroic mirror 4a has the optical characteristic that allows 5% of the laser beams, having the aforementioned wavelength, to transmit therethrough. Therefore, the human operator is capable of recognizing the laser beams by the IR camera 10.
Next, a description will be given with respect to operations to measure signals by using the EOS prober of FIG. 6.
When power voltage is applied to the wiring (or wires) of the IC wafer 1, an electric field is caused to occur and is applied to the electro-optical element 2. This causes a phenomenon in which a refractive index changes due to Pockel""s effect. Due to such a phenomenon, changes occur in polarization states between incoming beams and outgoing beams of the electro-optical element 2. That is, the laser beams radiated from the laser 8 propagate along certain paths and are incident on the electro-optical element 2 as the incoming beams, so that the incoming beams are reflected by the opposite surface facing with the IC wafer 1 and are returned back along the same paths as the outgoing beams, which are changed in polarization states. Such outgoing beams whose polarization states are changed due to the electric field applied to the electro-optical element 2 propagate back along the optical paths C, B, A in turn and are then input to the EOS optical system 6.
In the EOS optical system 6 receiving the beams whose polarization states are changed, changes of the polarization states are converted to changes of light intensities, which are detected by the photodiodes to produce electric signals. Then, the electric signals are processed by a signal processing portion (not shown). Thus, it is possible to measure signals applied to the wiring of the IC wafer 1.
The aforementioned example of the EOS prober needs illumination light to illuminate the IC wafer 1, which is a measured subject, in order to perform positioning of the IC wafer 1. Herein, the illumination light is used for the positioning of the IC wafer 1, so it is preferable to turn on the light source (i.e., halogen lamp 9) of the illumination light during measurement.
However, the aforementioned EOS prober is designed such that the optical paths of the light of the halogen lamp 9 coincide with the optical paths of the laser beams used for the measurement. In some case, the illumination light are introduced into the EOS optical system, which performs the measurement, as optical noise. So, there is a problem in which such optical noise deteriorates a S/N ratio in measurement of the signals.
It is an object of the invention to provide an electro-optic sampling prober that is capable of preventing illumination light causing optical noise from being input to photodiodes so that a S/N ratio is improved.
An electro-optic sampling prober of this invention is basically used to measure a waveform of a measured signal applied to wiring of an IC wafer. Herein, a laser radiates laser beams, which transmit through an optical fiber and are converted to parallel beams by a fiber collimator. The parallel beams are introduced into an optical module containing an optical isolator and photodiodes. Then, the parallel beams pass through an optical wavelength filter to propagate through a prober unit, which is constructed by a reflecting mirror, a dichroic mirror and a half mirror. The parallel beams are incident on an electro-optical element by way of an objective lens.
The electro-optical element is placed on the IC wafer and is changed in polarization state in response to an electric field being caused by the measured signal. The parallel beams are reflected by a surface mirror of the electro-optical element placed opposite to a surface of the IC wafer, so that reflected beams propagate back through the prober unit to input to the optical wavelength filter. Herein, the optical wavelength filter has an optical characteristic such that a center wavelength in transmission of light coincides with a wavelength of the laser beams whose intensities are maximal, so it is possible to prevent components of light, which are not required for measurement, from being unnecessarily returned to the optical module. The reflected beams passing through the optical wavelength filter is introduced into the optical module as returned beams, which are isolated by the optical isolator and are input to the photodiodes to generate electric signals.
During the measurement, a human operator watches an image of a selected portion of the IC wafer presently placed beneath the prober unit to adjust a positional relationship between the prober unit and IC wafer. In order to do so, the electro-optic sampling prober is equipped with a halogen lamp, an infrared camera and a monitor. Herein, the halogen lamp radiates illumination light, which propagate through the prober unit and is incident on the electro-optical element on the IC wafer by way of the objective lens. Then, the illumination light is reflected by the surface mirror of the electro-optical element and is supplied to the infrared camera via the prober unit. Incidentally, the dichroic mirror has a specific optical characteristic such that a transmission factor greatly changes between first and second wavelengths (e.g., 1330 nm and 1500 nm). That is, the dichroic mirror substantially transmits components of light whose wavelengths are smaller than the first wavelength and which are detected by the infrared camera while substantially preventing other components of light whose wavelengths are higher than the second wavelength from being transmitted therethrough.
Due to the aforementioned optical characteristics of the optical wavelength filter and dichroic mirror, it is possible to prevent components of light (i.e., components of the illumination light), which are not required for measurement, from being returned to the optical module. Thus, the photodiodes do not detect such xe2x80x9cun-requiredxe2x80x9d components of light, so it is possible to improve a SIN ratio in measurement.